In recent years, with the proliferation of personal computers into homes, digital still cameras (hereinafter referred to simply as digital cameras) that enable input of picture image information, such as photographed scenery, a portrait, or the like, into a personal computer are rapidly becoming more popular. Further, with the development of higher performance mobile cellular telephones, portable modular cameras for mounting in such cellular telephones and providing picture image input have also become more common.
An image pickup element, such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor), is used in these cameras. Advancements in the miniaturization of camera elements in recent years have allowed these cameras to be designed, as a whole, to be extremely miniaturized. Further, advancements in achieving a larger number of pixels in the same size image pickup element have required the development of higher optical performance from the lens used to form images onto such a higher resolution, image pickup element.
Japanese Laid-Open Patent Application H10-48516, Japanese Laid-Open Patent Application 2002-221659, and U.S. Pat. No. 6,441,971 describe exemplary imaging lenses for such cameras that include only three lens elements. Additionally, Japanese Laid-Open Patent Application 2002-517773 describes an imaging lens that includes only four lens elements. In the imaging lens described in Japanese Laid-Open Patent Application H10-48516, the stop is positioned between the second and third lens elements, in order from the object side. In the imaging lenses described in Japanese Laid-Open Patent Applications 2002-221659 and 2002-517773, the stop is positioned between the first and second lens elements in order from the object side. In the imaging lens described in U.S. Pat. No. 6,441,971, the stop is positioned on the object side of the first lens element in order from the object side. All of the imaging lenses described in the four patent documents discussed above have a lens element with a meniscus shape at the extreme object side of the imaging lens.
Advances in miniaturization and the ongoing trend of larger numbers of pixels in image pickup elements will increase the need for higher resolution and yet more compact imaging lenses, especially imaging lenses for digital cameras. Although in the past cost and compactness were the primary requirements for the imaging lens of a portable modular camera, as advancements continue in providing larger numbers of pixels in an image pickup element for use in portable modular cameras, the demand for improvement of optical performance has recently increased. More particularly, the next generation of portable modular cameras are expected to provide even higher optical performance with a larger number of pixels.
Accordingly, the development of a wide variety of imaging lenses based on a comprehensive consideration of cost, performance and compactness is required. The development of an imaging lens that is low in cost and which also has high performance is required, while satisfying the requirements of compactness that enable mounting the imaging lens in, for example, a portable modular camera. In addition, ample consideration must be given to optical performance so as to enable the imaging lens to be used in the next generation of portable modular cameras.
In response to such demands, a three-lens or a four-lens construction may be used in order to satisfy the requirements of compactness and low cost, and consideration can be given to the aggressive use of aspheric surfaces in order to achieve higher performance. In this case, an aspheric surface contributes to the compactness and higher performance. However, because the use of aspheric surfaces is disadvantageous from the manufacturing point of view and it may easily increase cost, sufficient consideration should be given to manufacturing issues before a particular design using aspheric surfaces is approved for mass production.
The imaging lens described in each of the patent documents discussed above includes aspheric surfaces in a three-lens-element construction or a four-lens-element construction. However, in order to achieve favorable optical performance, the compactness of the imaging lens becomes unfavorable. Generally, with an imaging lens having a three-lens-element construction, even if there is adequate optical performance for a portable modular camera, the performance is inadequate for use in the next generation of portable modular cameras or in digital cameras. Additionally, with an imaging lens having a four-lens construction, although the optical performance can be improved over a three-lens construction, the cost and size of the imaging lens increase.